The production of many metals or useful metal-containing compounds from ores, for example, tin from cassiterite, tantalum and niobium from tantalite/columbite ores, scandium from thortveitate, uranium from pitchblend or uraninite, and rare-earth elements from monazite or bastnasite, typically employs at some point in the production process or in the treatment of tailings therefrom, a hydrofluoric acid digestion, leach or the like. Such treatment is often necessary to convert the myriad of different components including insoluble minerals, metals or compounds thereof, including various radionuclides, such as their oxides, halides, carbonates, fluorides, phosphates, sulfates or the like to species which are soluble in aqueous systems such that they can be separated out by selective precipitations, extractions or the like. The insolubilities of these components are often magnified by the refractory or inaccessible crystalline nature or the material, i.e., the matrix in which these components are occluded, entrapped or chemically incorporated, thus necessitating the drastic and extensive HF treatment. Also, in many such processes, HF or other fluorine containing reactant is employed for converting one species to another, such as the conversion of various oxides of uranium to the highly soluble UF.sub.6. The result of these HF treatments is that somewhere in the processing operations, whether primary ore manufacture or tailings processing or the like, the massive and extremely deleterious HF levels must be neutralized, typically and conveniently with lime. Such neutralization, of course, produces large quantities of insoluble calcium fluoride waste sediment or sludge which either physically entraps or chemically incorporates into its crystal lattice residual but significant quantities of valuable metals including radionuclides such as uranium and thorium, as well as bringing down large valuable fluorine values. The typical fluorine values of these sediments by weight, dry basis, are from about 35% to about 45%, and for each of uranium, thorium, tantalum, and niobium about 0.1%. Greater or lesser concentrations of these elements, or different metal species may be found depending on the type of metal ore and processing operation involved. It is noted that the quantities of HF needed in various ore treatment operations around the world is enormous as well as expensive, and thus, fluorine values recovery is a very important consideration in any process directed to the treatment of ore, per se, or of ore processing tailings or waste material or streams.
In this regard many ores, per se, depending on their geographical sources, as well as the residual wastes from their processing into metals or compounds thereof naturally contain substantial fluorine values, for example, fluorite, cryolite, fluorapatite, sellalite, topaz, villiaumite and bastnasite contain fluorine values ranging up to about 50% by weight, and often contain substantial radionuclide values. It is noted that in many instances the initial ore may contain only relatively low concentrations of radionuclide materials but during processing of the ore by flotation, sedimentation, extraction, chemical precipitation, evaporation or the like operations, the level of radionuclide materials and also of fluorine values can become concentrated and raised substantially, e.g., assaying at least above about 30 pCi/g, and often much higher.
In recovering the various metal and non-metal components or values therefrom, the ore is typically digested in an acid such as H.sub.2 SO.sub.4, HCl, HNO.sub.3 or even HF and substantial fluorine values are liberated, usually as HF. Thus, the recovery of these fluorine values becomes an important consideration whether the values are naturally occurring in the ore or are man-made; but, incident thereto, a two-fold problem, discussed in detail below, exist. These problems which are associated with ore processing waste treatment operations derive from the fact that the fluorine values must be recovered in an economical manner while concurrently reducing the radionuclide levels therein and in final residue or discard to chemical landfill quality, and also, preferably while recovering the radionuclide values, particularly uranium as well as other metal values such as Mg which may be employed in the recovery operations.
For the ore treatment scenario discussed above, certain valuable metal ores come into the production plant with substantial radionuclide values, for example tantalum and niobium-bearing materials may contain up to about 1.0% by weight uranium and thorium, with associated daughter products in equilibrium. When the ores are digested, for example, with HF, the various radionuclide values typically do not mobilize into the liquor in an efficient and consistent manner. Therefore, the final ore processing residues or tailings contain substantial quantities of uranium and thorium as insoluble fluorides, and other insoluble metal fluorides such as scandium. The relatively high radionuclide content creates a large problem to the disposal of such residues in that their classification according to Federal Regulations is not chemical waste but hazardous nuclear waste. Typcially, the fluoride content in the aforementioned ore residues can range up to about 30% by weight or higher. Other typical metals which are present in the residues include: calcium, 15%, aluminum, 5%; zirconium, 2%; tantalum, 1.2%; niobium, 1.7%; and scandium, 0.15%, the percentages being on dry basis. These fluoride-bearing ore residues which often having radioactivity levels of well above 30 pCi/g, are usually impounded at the plant site until a determination can be made concerning their disposition.
As mentioned above and as is discussed hereinafter in greater detail, the relatively insoluble mineral fluoride matrices resulting from the treatment of ore residues and sediments described above typically have been treated heretofore with sulfuric acid at elevated temperatures. In this process, hydrofluoric acid is formed and liberated. With the fluoride removed as HF, the various metals within the matrix are converted to soluble sulfates which can then be dissolved in an aqueous solution and removed by extraction, precipitation, or other techniques. The purpose of such metal recovery is to generate revenue, e.g., from scandium, yttrium, or other valuable metals or to decontaminate solids, i.e., remove uranium, thorium or radium to obviate the premium charged for radioactive waste disposal.